I simply drove between mile markers and its fine.
I am still in the process of calibrating my gas mileage haven't done my first fill up yet.
Other than that I like the Ultra Gauge, only problem I have with it is that it is so insanely hot here that I have to unhook it off the windshield bracket and place it into the shaded speedometer area and then cover it with something like an envelope of junk mail.
One thing I wish it had was to be able to unhook the Gauge from the wire so that I could just take inside or stick it in the armrest console.
One thing I have learned from it already is why my gas mileage sucks in this area, HILLS.
Where I drive everyday is up & down small hills continuously, can rarely drive more than a mile in any direction without going up or down and it sucks down the gas going up hills no matter how slow you go.
It fits nicely in the lower left corner of windshield and took like 2 minutes to install start to finish.